1. Field
This invention relates to the field of methods for etching a substrate, and more specifically to creating nanoscale perforations in a carbon substrate for reverse osmosis desalinization.
2. Background
Desalinization is the process of removing salt from water, generally to produce fresh water suitable for human consumption or irrigation. Sea-going vessels require large quantities of fresh water for crew hydration as well as cleaning, systemic and industrial uses. While some fresh water can be stored, water tanks are bulky, unstable and take up space that could be used for other applications. Desalinization of available seawater would provide an abundant source of fresh water, limited only by the amount of energy required for the desalinization process.
One method of desalinization known in the art is the use of reverse osmosis filters. Reverse osmosis filters known in the art are porous hydrophilic polymer membranes. The membranes are characterized by nanoscale channels with a diameter calculated to allow the flow of water molecules while preventing the passage of dissolved salt ions, separating fresh water from saltwater.
Reverse osmosis filters, known in the art, can be up to 1 mm thick. This thickness requires high energy levels (above 1.8 kWh/m3) to force water molecules through for desalinization. This makes these polymer filters unsuitable for large-scale applications or in cases where a power supply may be limited, such as aboard a sea-going vessel.
Attempts have been made in the prior art to create thinner reverse osmosis filters. These attempts include utilizing a graphene layer with nanoscale perforations. Graphene is a term used to describe a layer of carbon that has the thickness of approximately one atom. Because a graphene layer is so thin, it requires considerably less energy for water flow than a polymer-constructed membrane.
However, is a recognized problem in the art that etching and masking techniques for constructing a filter structurally compromise the graphene layer and are difficult to control. Techniques which have been attempted in the prior art include electron/helium ion beam exposure, block copolymer masking, photolithography and chemical etching. Each of these methods has drawbacks.
Electron/helium ion beam exposure is slow and difficult to control. Block copolymer masking and photolithography utilize complex, rigidly predefined masking patterns which do not account for individual grain boundaries within the carbon layer and may result in structural instability. Block copolymer masking and photolithography also require that the mask be removed after creation of the perforation, requiring additional processing time and potentially damaging the filter. Chemical etching preferentially attacks carbon grain boundaries and defects, rendering the filter structurally unstable.
It is desirable to fabricate a carbon filter rapidly and controllably with perforations of a suitable scale, without inducing mechanical instability in the completed filter.